Structure and Method for Alignment Marks

ABSTRACT

The alignment mark and method for making the same are described. In one embodiment, a semiconductor structure includes a plurality of gate stacks formed on the semiconductor substrate and configured as an alignment mark; doped features formed in the semiconductor substrate and disposed on sides of each of the plurality of gate stacks; and channel regions underlying the plurality of gate stacks and free of channel dopant.

PRIORITY DATA

The present application is a continuation application of U.S. patentapplication Ser. No. 12/783,200, filed May 19, 2010, which isincorporated herein by reference in its entirety.

TECHNICAL FIELD

The present disclosure relates generally to alignment marks forlithographic processes and, more particularly, to a structure and methodfor an improved alignment mark for high-k metal gate processes.

BACKGROUND

Alignment marks are important in fabrication of semiconductor, orintegrated circuit (“IC”), devices because the devices are produced byaligning several layers of conductive, semiconductive, and insulativematerials one atop the other with reference to the alignment marks. Itis critical that each layer is precisely aligned with the previous layerso that the resultant circuits are functional and reliable. Typically,aligning a layer with a previous one is accomplished using a waferstepper, which is used to optically project a circuit pattern on a maskmounted therein onto a layer of the wafer disposed on a wafer chuck ofthe stepper. Before the mask pattern is transferred, the wafer mustfirst be precisely aligned with the mask. Once such alignment isachieved, the remaining steps of projecting the mask pattern on to thesemiconductor may be performed.

During the alignment phase, the position of the alignment mark on thewafer is typically sensed using a laser beam, which is bounced off thealignment mark to produce a reflective light signal. This reflectivelight is reflected back to an inspector of the stepper. The stepperanalyzes the reflected light to determine the exact position of thealignment mark. Notably, the quality of the signal reflected from thealignment mark is directly dependent on the reliability and integrity ofthe structure thereof. Alignment marks fabricated using existingtechnologies do not lead to a strong reflected signal, which makesaccurate alignment more difficult.

Therefore, while existing methods of forming alignment marks have beengenerally adequate for their intended purposes, they have not beenentirely satisfactory in every aspect.

SUMMARY

The alignment mark and method for making the same are described. In oneembodiment, a semiconductor structure includes a plurality of gatestacks formed on the semiconductor substrate and configured as analignment mark; doped features formed in the semiconductor substrate anddisposed on sides of each of the plurality of gate stacks; and channelregions underlying the plurality of gate stacks and free of channeldopant.

In another embodiment, a semiconductor structure includes asemiconductor substrate having a device region and an alignment region;a field-effect device formed within the device region; and an alignmentmark formed within the alignment region. The field-effect deviceincludes a first gate stack formed on the semiconductor substrate; afirst source and drain regions formed in the semiconductor substrate anddisposed on both sides of the first gate stack; and a first channelregion having a channel doped feature formed in the semiconductorsubstrate and underlying the gate stack. The alignment mark includes asecond gate stack formed on the semiconductor substrate; a second sourceand drain regions formed in the semiconductor substrate and disposed onboth sides of the second gate stack; and a second channel region free ofchannel doped feature in the semiconductor substrate and underlying thesecond gate stack.

The present disclosure also provides a method including providing asemiconductor substrate having a device region and an alignment region;performing a first ion implantation to the semiconductor substratewithin the device region while the alignment region is covered by animplantation mask layer; thereafter forming a first polysilicon gatestack in a device region and a second polysilicon gate stack in analignment region; and thereafter performing a second ion implantation tothe semiconductor substrate within the device region and within thealignment region.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure is best understood from the following detaileddescription when read with the accompanying figures. It is emphasizedthat, in accordance with the standard practice in the industry, variousfeatures are not drawn to scale. In fact, the dimensions of the variousfeatures may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 is a flowchart of a method for making a semiconductor structurehaving an alignment mark constructed according to various aspects of thepresent disclosure.

FIGS. 2-10 are sectional views of a semiconductor structure having analignment mark at various fabrication stages constructed according tovarious aspects of the present disclosure in various embodiments.

FIG. 11 is a top view of an alignment mark of the semiconductorstructure of FIG. 8 constructed according to various aspects of thepresent disclosure.

DETAILED DESCRIPTION

The present disclosure relates generally to alignment marks forlithographic processes and, more particularly, to a structure and methodfor an improved alignment mark for high-k metal gate lithographyprocesses. It is to be understood that the following disclosure providesmany different embodiments, or examples, for implementing differentfeatures of various embodiments. Specific examples of components andarrangements are described below to simplify the present disclosure.These are, of course, merely examples and are not intended to belimiting. In addition, the present disclosure may repeat referencenumerals and/or letters in the various examples. This repetition is forthe purpose of simplicity and clarity and does not in itself dictate arelationship between the various embodiments and/or configurationsdiscussed. Moreover, the formation of a first feature over or on asecond feature in the description that follows may include embodimentsin which the first and second features are formed in direct contact, andmay also include embodiments in which additional features may be formedinterposing the first and second features, such that the first andsecond features may not be in direct contact.

FIG. 1 is a flowchart of a method 100 for making a semiconductor deviceaccording to one embodiment. The semiconductor device includes a metalgate stack and an alignment mark constructed according to variousaspects of the present disclosure. FIGS. 2 through 10 are sectionalviews of a semiconductor structure 200 at various fabrication stages andconstructed according to various embodiments. The semiconductorstructure 200 and the method 100 of making the same are collectivelydescribed with reference to FIGS. 1 through 10.

Referring to FIGS. 1 and 2, the method 100 begins at step 102 byproviding a semiconductor substrate 210. The semiconductor substrate 210includes silicon. Alternatively, the semiconductor substrate 210includes germanium, silicon germanium or other proper semiconductormaterials. The semiconductor substrate 210 also includes variousisolation features such as shallow trench isolation (STI) formed in thesemiconductor substrate 210 to separate various devices. In oneembodiment, the semiconductor substrate 210 includes an alignment region212 for an alignment mark and a device region 214 for one or morefield-effect transistors (FETs) and or other devices. Various STIfeatures 216 are formed in the semiconductor substrate 210. Theformation of the STI features 216 includes etching a trench in asubstrate and filling the trench by one or more insulator materials suchas silicon oxide, silicon nitride, or silicon oxynitride. The filledtrench may have a multi-layer structure such as a thermal oxide linerlayer with silicon nitride filling the trench. In one embodiment, theSTI feature 216 is created using a process sequence such as: growing apad oxide, forming a low pressure chemical vapor deposition (LPCVD)nitride layer, patterning an STI opening using photoresist and masking,etching a trench in the substrate, optionally growing a thermal oxidetrench liner to improve the trench interface, filling the trench withCVD oxide, using chemical mechanical planarization (CMP) to etch back,and using nitride stripping to leave the STI features. The semiconductorsubstrate 210 also includes various n-wells and p-wells formed invarious active regions.

Still referring to FIGS. 1 and 2, the method 100 proceeds to step 104 byperforming a first ion implantation to introduce doping species in thesemiconductor substrate 210 in the device region 214 while the alignmentregion 212 is protected from the first ion implantation using an implantmask layer 217. The first ion implantation 218 includes one or more ionimplantations implemented before the formation of gate stacks to formvarious doped features 219. In one embodiment, the first ionimplantation 218 includes a well ion implantation to form a well, suchas a n-type well (n-well) or a p-type well (p-well), an ion implantationto adjust threshold voltage, an anti-punch through (APT) ionimplantation, or combinations thereof. A patterned photoresist layer orother suitable material layer, such as silicon nitride, is used as animplant mask layer 217. In one embodiment, a patterned photoresist isused as the implant mask layer 217 and is also referred to by 217. Thepatterned photoresist layer 217 is formed on the substrate to cover thealignment region 212 while the device region 214 is exposed or partiallyexposed through the openings of the patterned photoresist layer 217.Then an ion implantation process 218 is applied to the device region 214such that doping species form respective doped feature 219. In oneexample, when a p-type dopant is introduced to the semiconductorsubstrate to form one or more p-wells, the implant mask layer 217 ispatterned to cover the alignment region and a portion of the deviceregion for an n-well, then a p-type dopant is introduced to thesemiconductor substrate 210 by an ion implantation to form one or morep-wells in the device region 214. The patterned photoresist layer 217 isformed using a photolithography process including photoresist coating,soft baking, exposing, post-exposure baking (PEB), developing, and hardbaking. The patterned photoresist layer 217 is removed thereafter usinga suitable process, such as wet stripping or plasma ashing.Alternatively, if the hard mask layer is present, then the patternedphotoresist layer is used to pattern the hard mask layer and the patternhard mask layer is used as an implant mask.

Referring to FIGS. 1, 3 and 4, the method 100 proceeds to step 106 byforming gate stacks in the device region 214 and the alignment region212. In one embodiment, various gate material layers are formed on thesemiconductor substrate 210 as illustrated in FIG. 3. The gate materiallayers include a dielectric material layer 220 and a silicon layer 222,such as polycrystalline silicon (polysilicon). In the presentembodiment, the silicon layer 222 may be non-doped and the dielectricmaterial layer 220 includes a high-k dielectric material layer. Thesilicon layer 222 alternatively or additionally may include amorphoussilicon. The high-k dielectric material layer 220 includes a dielectricmaterial having the dielectric constant higher than that of thermalsilicon oxide, which is about 3.9. In one example, the high-k dielectriclayer 220 includes hafnium oxide (HfO). In various examples, the high-kdielectric layer 220 includes metal oxide, metal nitride, orcombinations thereof. In one example, the high-k dielectric layer 220includes a thickness ranging between about 10 angstrom and about 100angstrom.

In various embodiments, the gate material layers include multi-layerdielectric materials, such as an interfacial layer (e.g., silicon oxide)and a high-k dielectric material layer disposed on the interfaciallayer. In another embodiment, a hard mask layer 224, such as siliconnitride (SiN) or silicon oxide (SiO2), is further formed on the gatematerial layers for gate patterning. In various embodiments, theinterfacial layer may be formed by chemical oxide technique, thermaloxide procedure, atomic layer deposition (ALD) or chemical vapordeposition (CVD). The high-k dielectric material layer may be formed byCVD, ALD, plasma enhanced CVD (PE CVD), or plasma enhanced ALD (PEALD).The non-doped amorphous silicon or polysilicon layer 222 can be formedusing CVD with precursor silane (SiH4) or other silicon based precursor.The deposition of the non-doped amorphous silicon layer 222 can beperformed at a raised temperature. The hard mask layer (SiN or SiO2) canbe formed by CVD or other suitable technique.

Then the gate material layers are patterned to form one or moreresistors, such as gate stacks 226 and 228 in the alignment region 212,and one or more gate stacks (or dummy gates), such as a gate stack 229in the device region 214, as illustrated in FIG. 4. The patterning ofthe gate material layers can be achieved by a lithography process and/oran etch process. For example, a patterned photoresist layer is formed onthe hard mask layer 224 defining various resistor regions and gateregions, using a photolithography process including photoresist coating,soft baking, exposing, post-exposure baking (PEB), developing, and hardbaking. Then, the hard mask layer 224 is etched through the openings ofthe patterned photoresist layer, forming a patterned hard mask. The gatematerial layers are further etched using the patterned hard mask,forming the various resistors and gate stacks. The patterned photoresistlayer is removed thereafter using a suitable process, such as wetstripping or plasma ashing. Alternatively, if the hard mask layer is notpresent, then the patterned photoresist layer is directly utilized as anetch mask to etch the gate material layers.

The gate stacks 226 and 228 are configured to form an alignment mark. Inone embodiment, the gate stacks in the alignment region 212 areconfigured as a periodic structure to form a grating alignment mark. Forexample, the grating alignment mark includes two, three, four, or moregate stacks disposed in parallel and periodically. In anotherembodiment, the alignment mark includes a gate stack designed as asquare frame used by box in box or frame in frame alignmenttechnologies. In another embodiment, the gate stack 229 is formed in thedevice region 214 for a field-effect transistor (FET), such as ametal-oxide-semiconductor (MOS) transistor. The FET can be an n-typefield-effect transistor (nFET) or a p-type field-effect transistor(pFET). Alternatively, the gate stack 229 is formed in the device region214 for an imaging sensor.

Referring to FIGS. 1 5 and 6, the method 100 proceeds to step 108 byperforming a second ion implantation to introduce doping species intothe semiconductor substrate 210 in both device region 214 and thealignment region 212. The second ion implantation is implemented afterthe formation of the gate stacks (e.g. 226, 228 and 229) at step 106.The second ion implantation may include various implantation steps toform respective doped features. In one embodiment, the second ionimplantation includes light doped drain (LDD) ion implantation andheavily doped source and drain (S/D) implantation. Since the second ionimplantation is implemented after the formation of the gate stacks atstep 106, the corresponding doped features are substantially formed onsides of the gate stacks but not in the channel regions underlying thegate stacks.

In furtherance of the present embodiment, the second ion implantationincludes an LDD implantation 230 to form various LDD features 232 in thealignment region 212 and the device region 214, as illustrated in FIG.5. In one example, an n-type dopant, such as phosphorous or arsenic, isintroduced to the semiconductor substrate 210 in both the device region214 and the alignment region 212 to form n-type LDD features. The dopingdose is greater than about 1×10¹⁴/cm² to effectively change therefractive index of the semiconductor substrate 210. In one example, thesilicon substrate has a change of the refractive index from about 3.89to about 3.0 after the n-LDD implantation. In the alignment region 212,the difference in the refractive index between the silicon substrateunderlying the gate stack and LDD implanted silicon substrate canenhance the contrast of the alignment mark during the alignmentoperation. In one example, the LDD implantation includes a dose about10¹⁵ ions/cm². In another example, the LDD implantation includes animplant energy ranging between about 50 keV and about 100 keV. In oneembodiment, the photomask having a LDD implant pattern defines anadditional opening for the alignment region. For example, if thealignment region 212 has dimensions of 50 micron×882 micron, thecorresponding photomask has an additional opening of 50 micron×882micron for the alignment region 212 such that the LDD features areformed therein. In another embodiment, a p-type doping species, such asboron (B), may be alternatively used to form p-type LDD features in thealignment region 212.

The second ion implantation may further include heavily doped source anddrain (S/D) features formed by another ion implantation step after theLDD. Therefore each gate stack in the device region 214 and thealignment region 212 include both LDD features and S/D features,collectively referred to as source and drain regions. When the deviceregion 214 includes both n-type FETs (nFETs) and p-type FETs (pFETs),the source and drain regions are formed, respectively, for the n-typeFETs and the p-type FETs, using proper doping species.

In one embodiment, taking n-type FETs as an example, the LDD features232 are formed by an ion implantation with a light doping dose.Thereafter, sidewall spacers 234 are formed by dielectric deposition andanisotropic etch, such as plasma etch. Then the heavily doped S/Dfeatures are formed by an ion implantation 236 with a heavy doping dose.The various source and drain features of the p-type FETs can be formedin a similar procedure but with opposite doping type.

In one embodiment, during various doping processes to form variousn-type source and drain features, the corresponding features includingLDD and S/D are also formed in the alignment region 212, as illustratedin FIG. 6. The sidewall spacer 234 can be formed to the gate stacks inthe alignment region as well. In one embodiment, a high temperatureannealing process may be followed to activate the various doping speciesin the source and drain features in the device region 214. In anotherembodiment, the second ion implantation additionally or alternativelyincludes a pocket ion implantation using the dopant opposite from thatof the source and drain, and is formed in the substrate and adjacent thechannel region.

Referring to FIGS. 1, 7 and 8, the method 100 proceeds to step 110 byforming metal gates in the device region 214 and the alignment region212. In one embodiment, an inter-layer dielectric (ILD) layer 242 isfirst formed on the semiconductor substrate 210. The ILD layer 242includes silicon oxide, low k dielectric material, other suitabledielectric materials, or combinations thereof. In another embodiment,the ILD layer 242 includes a buffer silicon oxide layer, a contact etchstop layer (CESL) formed on the buffer silicon oxide layer, and anotherdielectric material layer disposed on the CESL. The formation of the ILDlayer 242 is described below.

The ILD layer 242 is formed by a suitable technique, such as CVD. Forexample, a high density plasma CVD can be implemented to form the ILDlayer 242. In one embodiment, the ILD layer 242 deposits on thesemiconductor substrate 210, and fills in the gaps between the gatestacks in the alignment region 212 and the gaps between the gate stacksin the device region 214. In furtherance of the embodiment, the ILDlayer 242 is formed on the substrate to a level above the top surface ofthe gate stacks, such as 226, 228 and 229. A chemical mechanicalpolishing (CMP) process is then applied to the ILD layer 242 to reducethe thickness of the ILD layer 242 such that the gate stacks are exposedfrom the top side. The processing conditions and parameters of the CMPprocess, including slurry chemical and polishing pressure, can be tunedto partially remove and planarize the ILD layer 242. The CMP process maypartially or completely remove the hard mask layer 224.

After the formation of the ILD layer 242, an etch process is applied toremove the polysilicon or amorphous silicon of the gate stack 229 withinthe device region 214 and the alignment region 212. If the hard mask ispresent and is not removed at the CMP step, the etch process removes thehard mask layer 224 as well. In one embodiment, the etch processincludes two steps where the first step is designed to remove the hardmask layer 224 and the second step is designed to remove the silicon inthe gate stacks in the device region 214 and the alignment region 212.After the silicon in the gate stacks is removed, trenches are resultedin the ILD layer 242 and are referred to as gate trenches.

In one embodiment, the first etch step to remove the hard mask layer 242may include phosphoric acid (H3PO4) solution, hydrofluoric acid (HF), orbuffered HF if the hard mask layer 224 include silicon nitride. Inanother embodiment, the etching process used to remove the polysilcionor amorphous silicon of the gate stacks may implement suitable dryetching, wet etching or combinations thereof. In one example, an etchingsolution including HNO3, H2O and HF, or NH4OH solution, may be used toremove polysilicon (or amorphous silicon). In another example, chlorine(Cl)-based plasma may be used to selectively remove the polysilicon.

After the formation of the gate trenches, one or more metal gatematerial layers are formed in the gate trenches. In one embodiment, ametal layer 246 of a proper work function (referred to as a workfunction metal) and a conductive layer 248 are filled in the gatetrenches. In one embodiment, the gate trenches in the device region 214and alignment region 212 are deposited with a work function metal layer246 and is then filled with the conductive material 248, forming a gateelectrode for a nFET. The work function metal 246 for the nFET isreferred to as a n-metal. The n-metal includes a metal-based conductivematerial having a work function compatible to the nFET. For one example,the n-metal has a work function of about or less than about 4.2 eV. Inone embodiment, the n-metal includes tantalum (Ta). In anotherembodiment, the n-metal includes titanium aluminum nitride (TiAlN). Inother embodiments, the n-metal includes Ta, TiAl, TiAlN, or combinationsthereof. The n-metal may include various metal-based film as a stack foroptimized device performance and processing compatibility. The n-metallayer can be formed by a suitable process, such as PVD. The conductivematerial layer 248 may include aluminum, tungsten or other suitablemetal. Then, a CMP process may be applied to remove the excessive workfunction metal and the conductive material. In one embodiment, thedevice region 214 includes both nFETs and pFETs. In this embodiment, themetal gates are formed for the nFETs and pFETs, respectively by a properprocedure. For example, after the removal of the silicon from thesilicon gate stacks, the metal gates for the nFETs and the alignmentmark are formed by a deposition for the n-metal layer, a deposition forthe conductive layer, and a CMP process to remove the excessive n-metallayer and the conductive layer while the pFETS are protected by apatterned photoresist layer. Then the metal gates for pFETs are formedby a deposition for the p-metal layer, a deposition for the conductivelayer, and a CMP process to remove the excessive p-metal layer and theconductive layer. Alternatively, a p-metal layer is deposited for thepFETs while the nFETs are protected by a patterned photoresist layer. An-metal layer is deposited for the nFETs and the alignment mark whilethe pFETs are protected by a patterned photoresist layer. Then aconductive layer is deposited to fill gate trenches for nFETs, pFETs andthe alignment mark. A CMP process is applied to the substrate to removethe excessive portion of the n-metal layer, p-metal layer, andconductive layer, forming the metal gates for nFETs, pFETs and alignmentmark.

The p-metal includes a metal-based conductive material having a workfunction compatible to the pFET. For one example, the p-metal has a workfunction of about or greater than about 5.2 eV. In one embodiment, thep-metal includes titanium nitride (TiN) or tantalum nitride (TaN). Inother embodiments, the p-metal include TiN, tungsten nitride (WN),tantalum nitride (TaN), or combinations thereof. The p-metal may includevarious metal-based film as a stack for optimized device performance andprocessing compatibility. The p-metal layer can be formed by a suitableprocess, such as physical vapor deposition (PVD), CVD, ALD, PECVD, PEALDor spin-on metal. The conductive material thereafter substantially fillsin the gate trench. The conductive material includes aluminum ortungsten according to various embodiments. The method to form theconductive material may include PVD, CVD, ALD, PECVD, PEALD or spin-onmetal. Then, a CMP process may be applied to remove the excessive workfunction metal and the conductive material, forming the metal gate.Although the semiconductor structure 200 only illustrates onefield-effect transistor in the device region 214, a plurality of FETsand other devices can be formed in the device region. The presentprocess to form the metal gates may have other alternative embodiment.For example, the metal gate for nFETs and pFETs may be formed by othersequence or other procedure.

In one embodiment, the metal gates may include a step to deposit ahigh-k dielectric material layer 244 the silicon oxide layer 220 in thegate trenches, and then a work function metal layer and a conductivelayer are formed on the high-k dielectric material layer 244. Thisprocess is referred to as high-k last process. Alternatively, in thehigh-k last process, the silicon oxide layer 220 is first removed beforeforming the work function metal layer and conductive material layer. Inthis case, a new interfacial layer, such as silicon oxide, is firstformed on the semiconductor substrate 210, then the high-k dielectricmaterial layer, work function metal layer and conductive material layerare formed in the corresponding gate trenches.

As described above, the alignment mark in the alignment region 212including the gate stacks 226 and 228 in the alignment region 212 arereplaced by a metal gate. Particularly, the polysilicon in the gatestacks 226 and 228 are replaced to form metal gates similar to the metalgate for the n-FETs in the device region 214 and formed in the sameprocess to form the metal gates for the nFETs. Therefore, the gatestacks in the alignment region 212 include the n-metal layer and theconductive material layer. In another example, the polysilicon gatestacks 226 and 228 may be replaced to form metal gates similar to themetal gates for the p-FETs in the device region 214 and formed by thesame process to form the metal gates for the pFETs. In this case, thegate stacks in the alignment region 212 include the p-metal layer andthe conductive material layer.

In another embodiment, the gate stacks for the alignment mark in thealignment region 212 remain as polysilicon gate stacks withoutreplacement. In this case, the alignment region 212 is covered by apatterned mask layer such as a patterned photoresist layer or apatterned hard mask layer during the gate replacement to form metalgates for devices in the device region 214, as illustrated in FIG. 9.

Referring to FIG. 10, the method 100 may proceed to step 112 by formingcontact holes to electrical interconnection. In one embodiment, acontact etch stop layer (CESL) 250 is formed on the ILD layer 242 andanother ILD layer 252 is formed on the CESL 250. Then a photoresistlayer (not shown) is coated on the semiconductor structure 200 in alithography process, a soft baking may be applied to the coatedphotoresist layer. Then a photomask (or mask) having a contact patternis placed on the lithography exposure apparatus and the semiconductorstructure 200 is secured on a wafer stage of the lithography exposureapparatus. Then the photomask is aligned to the semiconductor structure200 before exposing the coated photoresist layer. The alignmentoperation uses the alignment mark including the gate stacks 226 and 228in the alignment region 212. The alignment mark is further describedwith additional reference to FIG. 11. FIG. 11 is a top view of analignment mark of the semiconductor structure of FIG. 8 constructedaccording to various aspects of the present disclosure. In FIG. 11, analignment mark is labeled with numeral 260. The alignment mark 260 isformed in the alignment region 212 of FIG. 8. The alignment mark 260includes gate stacks 226 and 228, and may include additional gate stacksconfigured as a grating alignment mark. In one example, the gate stacksinclude a width of about 1.6 micron and a pacing of about 1.6 micron.

In yet another embodiment, the alignment mark 260 may include a secondset of gate stacks configured to a similar grating structure and usedfor alignment in a perpendicular direction. In one embodiment, thesecond set of gate stacks may be oriented in a direction perpendicularto the direction of the gate stacks 226 and 228. In another embodiment,the second set of gate stacks may be oriented in the same direction butwith a different grating pitch.

As the substrate 210 is doped by one or more doping processes, such asLDD doping, heavily doped S/D, and/or pocket implant, the refractiveindex of the substrate 210 is changed, the alignment signal issubstantially increased. A wafer quality (WQ) is defined toquantitatively describe the alignment signal quality. WQ is percentageof actual signal strength with reference to signal generated by fiducialmark. In one embodiment, WQ is defined as

WQ=(SS_(align)/Gain_(align))/(SS_(ref)/Gain_(ref))

Where SS_(align) is the signal strength of the alignment signal from thealignment mark, Gain_(align) is the signal strength of the gain of thealignment signal, SS_(ref) is the signal strength of the referencesignal from the fiducial mark, and Gain_(ref)) is the gain of thereference signal.

Usually, WQ should be more than 1% in order to obtain a reliablealignment results. In one example, the existing structure of thealignment provides a WQ less than 1%, such as 0.3%. In the disclosedstructure of the alignment mark, the WQ is increased to be greater than1%. In another example, for the alignment light of wavelength of about633 nm, the WQ is greater than 3. In yet example, for the alignmentlight of wavelength of about 532 nm, the WQ is greater than 8.

In the alignment operation, the semiconductor structure 200 (or wafer)is positioned by the wafer stage to align between the photomask and thewafer by utilizing the disclosed alignment structure. After thealignment, the coated photoresist layer is exposed. Other steps in thelithography process, such as post exposure baking (PEB), developing andhard baking, may follow to form the patterned photoresist layer with thecontact pattern aligned with the other features (gate stacks, source anddrain features) in the semiconductor structure. An etch process isapplied to the ILD layers 242 and 252 and form contact holes in the ILDlayers. Other processing steps may be subsequently implemented. Inanother embodiment, a conductive material, such as tungsten, is filledin the contact holes to form contacts. In one embodiment, a silicide isfirst formed on the semiconductor substrate 210 to reduce the contactresistance. The conductive material is thereafter filled in the contactholes to form contacts. A CMP process may subsequently implemented toremove the excessive conductive layer.

Although not shown, other alternative features and processing steps maybe present. In one embodiment, the device region 214 includes otherdevices, such as a static random access memory (SRAM) cell. In oneexample, a SRAM cell includes nFET and pFET configured as a crosscoupled two inventers, and may further includes other FET as pass gates.In another embodiment, the FET may be configured and designed for otherapplications, such as imaging sensors. In another embodiment, thesemiconductor substrate 200 includes more than one alignment regions.For example, each field may include an alignment mark. During theexposing process, each field is aligned with the photoresist using thealignment mark in the corresponding field. The field is exposed by thelithography light. The same process is repeated to the other fields ofthe wafer. In another embodiment, two or more alignment marks are formedin a different location of the wafer, the alignment is achievedaccording to an average among the alignment inputs various alignmentmarks. Then, the whole wafer is step-scanned or step-exposed. In anotherembodiment, the nFETs in the device region 214 include p-wells and pFETsin the device region 214 include n-wells. In another embodiment, thedevice region 214 includes p-cell and n-cell ion implantation featuresfor SRAM devices. The p-cell and n-cell ion implantation features aresimilar to the p-well and n-wells but have different doping doses andconcentrations. In another embodiment, the alignment mark is formed onsemiconductor substrate 210. For example, the alignment mark includes aplurality STI features formed in the alignment region and configured asa grating similar to the alignment mark 260 of FIG. 11 in configuration.

In another embodiment, the p-metal layer and n-metal layer are formed indifferent order such that n-metal layer is formed first and the p-metallayer is formed thereafter. In another embodiment, a pFET has a strainedstructure for enhanced carrier mobility and improved device performance.In furtherance of the embodiment, silicon germanium (SiGe) is formed inthe source and drain regions of the pFET to achieve a proper stresseffect. In one example of forming such a strained pFET, the siliconsubstrate within the source and drain regions of the pFET are recessedby one or more etching step. Then SiGe is epi grown in the recessedregions and heavy doped source and drain are formed in the epi grownSiGe features. In another example, a dummy spacer is formed after theformation of the LDD features. The dummy spacer is removed after theformation of the SiGe features. Then a main spacer is formed on thesidewalls of the associated gate stack, with a different thickness suchthat the heavy doped source and drain have an offset from the SiGefeatures. For instance, the main spacer is thicker than the dummy spacersuch that the heavy doped source and drain are formed in the SiGefeatures.

In another embodiment, a nFET has a strained structure for enhancedcarrier mobility and improved device performance. In furtherance of theembodiment, silicon carbide (SiC) is formed in the source and drainregions of the nFET to achieve a proper stress effect. The strained nFETcan be formed similarly as the strained pFET is formed. In anotherembodiment, the n-metal and p-metal layers each may include other propermetal or metal alloy. In another embodiment, the n-metal and p-metallayers each have a multi-layer structure to optimize work function andreduce threshold voltage.

Other processing steps may be implemented before, during and/or afterthe formation of the gate stacks (e.g. 226 and 228). For example, themultilayer interconnections are further formed after the step 112. Themultilayer interconnection includes vertical interconnects, such asconventional vias and horizontal interconnects, such as metal lines. Thevarious interconnection features may implement various conductivematerials including copper, tungsten and silicide. In one example, adamascene process is used to form a copper related multilayerinterconnection structure.

In one example, the high-k dielectric material layer can be formed byother suitable process such as metal organic chemical vapor deposition(MOCVD), or molecular beam epitaxy (MBE). In one embodiment, the high-kdielectric material includes HfO2. In another embodiment, the high-kdielectric material includes Al2O3. Alternatively, the high-k dielectricmaterial layer includes metal nitrides, metal silicates or other metaloxides. In another example, an interfacial layer, such as silicon oxide,is formed on the semiconductor substrate by a thermal oxidation, ALD,UV-Ozone Oxidation or other suitable method. In another example, acapping layer may be interpose between the high-k dielectric materiallayer and the n-metal (or p-metal) layer.

In a further embodiment as noted above, a high-k dielectric materiallayer can be formed in the gate stacks after the removal of thepolysilicon layer. For example, the dielectric material layer 220 formedat step 106 of FIG. 1 includes only silicon oxide layer as a dummy oxidelayer, then the high-k metal gate (HKMG) stack is formed by a high-klast procedure where both a high-k dielectric material layer and metallayer(s) are formed to fill the gate trench. Thus, formed HKMG stack isalso referred to as a complete replacement gate.

In another example, the formation of STI may include etching a trench ina substrate and filling the trench by insulator materials such assilicon oxide, silicon nitride, or silicon oxynitride. The filled trenchmay have a multi-layer structure such as a thermal oxide liner layerwith silicon nitride filling the trench. In one embodiment, the STIstructure may be created using a process sequence such as: growing a padoxide, forming a low pressure chemical vapor deposition (LPCVD) nitridelayer, patterning an STI opening using photoresist and masking, etchinga trench in the substrate, optionally growing a thermal oxide trenchliner to improve the trench interface, filling the trench with CVDoxide, and using chemical mechanical planarization (CMP) to etch back.

The various patterning process may include forming a patternedphotoresist layer by a photolithography process. An exemplaryphotolithography process may include processing steps of photoresistspin-on coating, soft baking, mask aligning, exposing, post-exposurebaking, developing photoresist and hard baking. The photolithographyexposing process may also be implemented or replaced by other propermethods such as maskless photolithography, electron-beam writing,ion-beam writing, thermal lithography, and molecular imprint.

The present disclosure is not limited to applications in which thesemiconductor structure includes a FET (e.g. MOS transistor) or SRAM,and may be extended to other integrated circuit having a metal gatestack and the alignment mark. For example, the semiconductor structuresmay include a dynamic random access memory (DRAM) cell, an imagingsensor, a capacitor and/or other microelectronic devices (collectivelyreferred to herein as microelectronic devices). In another embodiment,the semiconductor structure includes FinFET transistors. Of course,aspects of the present disclosure are also applicable and/or readilyadaptable to other type of transistor, including single-gatetransistors, double-gate transistors and other multiple-gatetransistors, and may be employed in many different applications,including sensor cells, memory cells, logic cells, and others.

It is understood that various different combinations of the above-listedembodiments and steps can be used in various sequences or in parallel,and there is no particular step that is critical or required.Furthermore, features illustrated and discussed above with respect tosome embodiments can be combined with features illustrated and discussedabove with respect to other embodiments. Accordingly, all suchmodifications are intended to be included within the scope of thisinvention.

What is claimed is:
 1. A device comprising: a first gate stack disposedover a semiconductor substrate and configured as an alignment mark; anddoped feature disposed in the semiconductor substrate and associatedwith the first gate stack, wherein the doped feature has a refractiveindex of about
 3. 2. The device of claim 1, wherein the doped featureincludes a p-type dopant.
 3. The device of claim 1, wherein the dopedfeature includes an n-type dopant.
 4. The device of claim 1, furthercomprising a second gate stack disposed over the semiconductorsubstrate; a doped well feature formed of a well dopant disposed in thesemiconductor substrate under the second gate stack; and a channelregion disposed in the substrate under the first gate stack and free ofthe well dopant.
 5. The device of claim 1, wherein the first gate stackincludes a high-k dielectric layer and a polysilicon material layerdisposed over the high-k dielectric layer.
 6. The device of claim 1,wherein the first gate stack includes a high-k dielectric layer and ametal layer disposed over the high-k dielectric layer.
 7. The device ofclaim 6, wherein the metal layer includes a n-type work function metallayer.
 8. The device of claim 6, wherein the metal layer includes ap-type work function metal layer.
 9. A device comprising: asemiconductor substrate having a device region and an alignment region;a first gate stack disposed over the semiconductor substrate in thedevice region; a first source/drain region formed in the semiconductorsubstrate adjacent the first gate stack; a second gate stack disposedover the semiconductor substrate in the alignment region; and a secondsource/drain region formed in the semiconductor substrate adjacent thesecond gate stack, wherein the second source/drain region has arefractive index of about
 3. 10. The device of claim 9, wherein thesecond source/drain region includes a lightly doped drain feature. 11.The device of claim 9, further comprising a third gate stack disposedover the semiconductor substrate in the alignment region, wherein thesecond source/drain region is adjacent the third gate stack.
 12. Thedevice of claim 11, further comprising a third source/drain regionformed in the semiconductor substrate adjacent the third gate stack,wherein the third source/drain region has a refractive index of about 3.13. The device of claim 9, wherein the first gate stack includes a firstdielectric material layer and a metal layer disposed over the high-kdielectric material layer; and the second gate stack includes a seconddielectric layer and a polysilicon layer disposed over the seconddielectric layer.
 14. The device of claim 9, wherein the secondsource/drain region includes a p-type dopant.
 15. The device of claim 9,wherein the second source/drain region includes an n-type dopant.
 16. Amethod comprising: providing a semiconductor substrate having a deviceregion and an alignment region; and modifying a refractive index of thesemiconductor substrate in the alignment region to increase a waferquality of an alignment signal from the alignment region to be greaterthan 1%.
 17. The method of claim 16, further comprising forming a firstgate stack over the device region and a second gate stack over thealignment region prior to modifying the refractive index of thesemiconductor substrate in the alignment region.
 18. The method of claim17, further comprising performing a gate replacement process on thesecond gate stack after modifying the refractive index of thesemiconductor substrate in the alignment region.
 19. The method of claim16, wherein modifying the refractive index of the semiconductorsubstrate in the alignment region includes performing an ionimplantation process to form a doped feature having a refractive indexof about
 3. 20. The method of claim 16, further comprising performing awell ion implantation process in the device region of the semiconductorsubstrate prior to modifying the refractive index of the semiconductorsubstrate in the alignment region.